The fluidity of membranes from normal and transformed cell lines was probed with the aid of spin-labeled stearic acid analogs. The following cells were examined: chick embryo fibroblasts transformed with Rous sarcoma viruses (Bryan high titer, Schmidt-Rupin, Temperature sensitive Bryan high titer) and Balb/3T3 mast cells transformed with murine sarcoma virus and SV40 virus. In contrast to previously published reports, no differences were observed between the fluidity of membranes from normal and transformed cells.